The long term objective is to define and characterize the developmental mechanisms which direct the correct anatomical and functional organization of the mammalian neocortex and its connections. The neocortex processes visual, auditory and somatic sensation giving rise to perception, volitional motor responses, and to more complex phenomena such as learning and memory. These functions are performed by the functionally specialized areas of the neocortex, each characterized by unique connectivity and architecture. Thalamocortical input play a fundamental role in defining neocortical areas: in adult, the modality of thalamic input dictates an area's functional identity. During development, thalmocortical afferents are required for the proper differentiation of neocortical areas. A crucial issue in both cortical function and development is defining the mechanisms that control the pathfinding of thalamocortical axons from dorsal thalamus to neocortex, and the subsequent area-specific targeting of these axons to their appropriate cortical areas. The first three aims address these issues by testing hypotheses on mechanisms and molecules involved in the pathfinding and area-specific targeting of thalamocortical axons. Our goal is to define roles of attractant and repellant axon guidance molecules, including the chemoattractant Netrin-1 and the chemorepellant Sema III/D, in pathfinding of thalamocortical axons, and the roles of the regulatory genes EMx1, Emx2, and Pax6 in controlling the development of area-specific thalamocortical projections. The fourth aim addresses hypotheses on the roles of Emx1, Emx2, and Pax6 in the genetic regulation of area identity of cortical output neurons in layers 6 and 5, focusing on their area-specific projections to targets in dorsal thalmus, or in the midbrain, hindbrain, and spinal cord, respectively. To accomplish these aims we will use complementary approaches, including in vivo and in vitro experiments to characterize axon guidance activities, loss of function analyses using knockout mice deficient for genes hypothesized to be involved, and gain of function studies using recombinant adenoviruses to express these genes in embryonic forebrain. Due to the conserved structural and functional homology of genes between rodents and humans, the mechanisms identified in the rodent model are anticipated to be similarly involved in human development.